Curable silicone compositions are applied to substrates to aid in the release of adhesive materials thereon. Laminates comprising a release coated paper or polymeric film with a pressure sensitive adhesive and a sheet material that can be a decorative lamina or label are used by stripping off the release liner, which is discarded, and affixing the lamina or label onto a surface.
Typically these release compositions cure by one of two mechanisms, thermal curing or photo-catalytic curing. Thermally curing release systems generally are comprised of the following compositions:
(A) a linear alkenyl substituted polysiloxane polymer that is the primary component or base polymer of the curable composition;
(B) a hydride functional cross-linking silicone, typically a methyl hydrogen siloxane polymer, copolymer or oligomer;
(C) an addition cure hydrosilylation catalysts, typically either a platinum or rhodium based catalyst;
(D) a cure inhibiting compound or mixtures thereof to increase the useful life of the coating bath.
The alkenyl functional silicone polymer release compositions typically used fall into one of two categories:
1) a linear alkenyl chain-stopped polymer:MviDxMvi  4)where Mvi indicates an alkenyl chain-stopping M group or
2) multi-functional alkenyl copolymers:MviDxDviyMvi  5)where Dvi indicates an alkenyl substituted D group. The alkenyl chain stopped polymers, MviDxMv, generally cure faster than the multi-functional copolymers, MviDxDviyMvi. As release composites are delaminated, the formulations based on the linear alkenyl chain-stopped polymers show significant increases in the delaminating force necessary as delaminating speed increases. In contrast, while the multi-functional alkenyl polymers tend to have a slower curing speed the increase in delaminating force with increasing delaminating speed is not nearly as great proportionately.
While the general practice usually employs linear base polymers, (A), solventless, high solids content formulations have been described. As described in U.S. Pat. No. 4,448,815 ('815) a linear alkenyl siloxane base copolymer is a copolymer of:(1) RcRd1Si(4-c-d)/2  1)where R is generally an alkyl radical, R1 is a low molecular weight olefinic substituent such as vinyl or allyl, c has value from 0 to 2 and the average of value of the sum c+d is 0.8 to 3; and(2) RnSiO(4-n)/2  2)where R is generally an alkyl radical and n has a value of 0.8 to 2.5. The preferred base copolymer of the '815 patent has the following linear structure:(H2C═CH)R2Si—O—(R2Si—O—)i—(RR1Si—O—)j—SiR2(H2C═CH)where the subscripts i and j are integers.
U.S. Pat. No. 4,774,111 ('111) describes a variation of the above linear copolymer where the R group in formula 2 is selected from alkyl and alkenyl radicals. The polymer of the '111 patent is defined as being substantially linear, i.e. having no more than a trace amount of T or Q groups. This substantially linear requirement for alkenyl functional heat curing silicone release compositions is repeated in U.S. Pat. Nos. 4,772,515; 4,783,552 and 5,036,117.
In contrast, the possibility of branched alkenyl polymers is admitted by the structural formulas recited in U.S. Pat. No. 4,057,596 ('596). In the '596 patent the composition comprises:
(A′) a substantially linear vinyl chain stopped polymer;
(B′) a linear methyl hydrogen polymer;
(C′) a methyl vinyl polysiloxane having at least three vinyl groups per molecule;
(D′) a methyl hydrogen polysiloxane having at least three hydride hydrogen atoms per molecule; and
(E′) a platinum hydrosilylation catalyst.
Component (C′) is described in the '596 patent as containing (H2C═CH)(CH3)SiO2/2 (Dvi), (H2C═CH)(CH3)2SiO1/2 (Mvi), and (H2C═CH)SiO3/2 (Tvi), units either singly or in combination with (CH3)2SiO2/2 (D), (CH3)3SiO1/2 (M), and (CH3)SiO3/2 (T). The optional inclusion of vinyl substituted T units and methyl T units permits the composition of the '596 patent to possess branched structures.
U.S. Pat. No. 4,386,135 describes a terminally unsaturated silicone polymer having the formulaR4-aSi((R2SiO—)bOSiR2R2)a  3)where a may be 2, 3, or 4. When a=4 the formula produces a Q resin. When a=3, a T structure results and the structure possesses only a single branch point. When a=2, the formula devolves to an alkenyl chain stopped linear polymer.
U.S. Pat. No. 5,468,826 teaches adhesion promoting additives comprised of organosiloxane copolymers having SiO4/2 units, alkoxy functional groups, epoxy or acryloxyalky groups and silicon hydride functionality. To those skilled in the art, it can be readily recognized that these have the disadvantage of increasing the release force during the delaminating process due to the resinous nature of the additive preventing low release force coatings from being available.
U.S. Pat. No. 3,873,334 teaches adhesion promoting additives comprised of acyloxy functional silanes, which additionally have silicon hydride or alkenyl functionality respectively. However, the acyloxy group liberated has the disadvantages of inhibiting addition cure, therefore slowing the addition curing process; liberating corrosive and objectionable odor hydrolysis products during the coating process. Furthermore, the acyloxy groups remaining in the release coating hydrolyzed over resulting in an undesirable interaction with the adhesive thus leading to undesirable delaminating release properties.
U.S. Pat. No. 5,567,764 teaches alkoxy containing alkenyl functional organopolysiloxanes as adhesion promoters for release coating onto polymeric films.
European patent 057984A2 teaches meth(acryl)oxy functional alkoxysilanes as adhesion promoters for release coatings on polymeric films.
Despite the above cited patents there still remains a need in the industry for release coating compositions, adhesion promoting additives which address the disadvantages of either stable adhesion to both paper and polymeric films, exhibit non-inhibiting effect on the curing, not liberate corrosive hydrolysis products, not exhibit objectionable odor during manufacturing of the laminate construction, not have hydrolysis products that would adversely interact with the adhesive used in the laminating construction, and a cost effective reproducible method of manufacture.